This invention is directed to a precision coining method particularly useful in coining the slow-wave structure of a traveling-wave tube and to the resulting coined helix assembly.
In electron beam tubes of the traveling-wave type, a stream of electrons in an electron beam is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic wave energy. In order to achieve the desired interaction, the electromagnetic wave is propagated along a slow-wave structure, such as an electrically conductive helix wound around the path of the electron beam. The slow-wave structure provides a path of propagation for the magnetic wave which is considerably longer than the axial length of the structure so that the traveling wave may be made to effectively propagate at nearly the velocity of the stream of electrons in the electron beam. Slow-wave structures of the helix type are usually supported within an encasing barrel by means of a plurality of (usually three) equally circumferentially spaced electrically insulating rods positioned around the helix and within the barrel.
One prior method of mounting the helix with its slow-wave structure and support rods within the barrel has been to triangulate the barrel. Initially the barrel is circular in section and it is thereupon distorted by applying forces to three points around its circumference to alter its section from circular toward triangular. When distorted in that manner, the helical slow-wave structure with its support rods is inserted into the distorted barrel. Upon removal of the distorting force, the barrel resiliently returns toward its original shape and, in doing so, compresses the rods and the slow-wave structure into a rigid assembly. This is described in U.S. Pat. No. 2,943,228 to Bernard Kleinman. An improvement thereon is disclosed in U.S. Pat. No. 3,514,843 to George Cernik.
Another way of achieving the compressed helix assembly is to mount the support rods on the helix of the slow-wave structure and retain that portion at room temperature or below. The metallic barrel has an initial inside diameter which is smaller than the circumscribing circle around the support rods so that, if assembled with those dimensions, there would be an interference fit. The barrel is heated and it is made of a material, such as copper, which expands upon heating. When an inside diameter is reached which is sufficiently large to receive the helix with its support rods, the helix and its support rods are inserted therein. Upon cooling, the barrel reduces in size to embrace the helix with its support rods in an interference fit.
Increasing frequency and reduction in wavelength at which the traveling-wave tubes operate have resulted in requirements for smaller slow-wave structures. The above-described methods for securing the slow-wave structure in its barrel are useful when the structures are of larger size. However, with reduction in size due to shorter wavelength, the prior assembly methods of triangulation and heat shrinking have not been completely satisfactory for assembling the helical slow-wave structure and its support rods into the barrel.